How X509CRLStreamWriter Works

Introduction

This document describes in detail how X509CRLStreamWriter modifies an existing X509 CRL in a streaming fashion. Before proceeding, it is useful to have an understanding of Abstract Syntax Notation 1, DER versus other forms of encoding for ASN1, and the CRL specification in RFC 5280ASN1 is a method for encoding data. Data exists in tag-length-value (TLV) units. The tag defines the type of data (e.g. an integer, a string), the length is the length of the data in bytes, and the value is the binary data.

For example the string “hello” would be encoded 160568656C6C6F in hexadecimal where 16 denotes the data is a IA5String (ASCII), 05 is the length of the data, and the subsequent bytes are the word “hello” encoded in ASCII. ASN1 data can be encoded in different formats, but the only one we are interested in is Distinguished Encoding Rules (DER) that define a single unambiguous way to encode each piece of ASN1. For the rest of this document, when I refer to ASN1 it is with the implicit understanding that it will be encoded using DER.

The eighth and seventh bits (where the first bit is the rightmost bit) of the tag byte describe the class of the tag. For our purposes, the only class we care about is the “universal” class which is the class of types built in to ASN1. The sixth bit describes whether the type is primitive or constructed. Primitive types are irreducible while constructed types can hold additional ASN1 items within themselves. One of the most important constructed types is the sequence. The final five bits (hereafter referred to as the tag number) are a number that maps to a specific data type.

To illustrate how a tag and tag number are related, let’s consider the sequence type. According to the X690 specification, the tag number for a sequence is 16. In binary that translates to 10000. A sequence is in the universal class which according to the specification makes the class bits (bits seven and eight) 00. Finally, a sequence is a constructed type, making the sixth bit a 1. Thus, the final tag is 00110000 in binary or 30 in hexadecimal.

Starting with with the universal types, we can build more and more complex TLVs to represent our data. For example, below is the schema specification for an X509 CRL as defined by RFC 5280.

CertificateList ::= SEQUENCE  {
     tbsCertList          TBSCertList,
     signatureAlgorithm   AlgorithmIdentifier,
     signatureValue       BIT STRING
}

TBSCertList ::= SEQUENCE  {
     version                 Version OPTIONAL,
                              -- if present, MUST be v2
     signature               AlgorithmIdentifier,
     issuer                  Name,
     thisUpdate              Time,
     nextUpdate              Time OPTIONAL,
     revokedCertificates     SEQUENCE OF SEQUENCE  {
          userCertificate         CertificateSerialNumber,
          revocationDate          Time,
          crlEntryExtensions      Extensions OPTIONAL
                                   -- if present, version MUST be v2
     }  OPTIONAL,
     crlExtensions           [0]  EXPLICIT Extensions OPTIONAL
                                   -- if present, version MUST be v2
}

AlgorithmIdentifier ::= SEQUENCE  {
     algorithm    OBJECT IDENTIFIER,
     parameters   ANY DEFINED BY algorithm OPTIONAL
}

Version ::= INTEGER  {  v1(0), v2(1), v3(2)  }

Time ::= CHOICE {
     utcTime        UTCTime,
     generalTime    GeneralizedTime
}

CertificateSerialNumber ::= INTEGER

Extensions ::= SEQUENCE SIZE (1..MAX) OF Extension

Extension ::= SEQUENCE  {
     extnID      OBJECT IDENTIFIER,
     critical    BOOLEAN DEFAULT FALSE,
     extnValue   OCTET STRING
                 -- contains the DER encoding of an ASN.1 value
                 -- corresponding to the extension type identified
                 -- by extnID
}

The specification looks complicated but it boils down to the CRL consisting of a block of initial meta-data, a sequence of items representing revoked certificates, a sequence of optional extensions (some of which are not optional as we will see later), an identifier indicating what cryptographic algorithm was used to generate the digital signature, and finally a digital signature to prevent the list from being tampered with. I will be referring to the elements of this specification extensively throughout the remainder of this document.

A digital signature works by taking a document and reducing it to a unique number (a hash) using a cryptographic hash function. The hash is then encrypted with a private key. To check the signature, one uses the public key of the signer to decrypt the signature and reveal the hash. The decrypted hash is then compared with the actual hash of the document received. If the hashes do not match, the document has been tampered with. The encrypting of the hash prevents anyone from tampering with the document and then modifying the hash to align with the alterations. A signature algorithm therefore is a combination of a hash algorithm and an encryption algorithm.

Standard Reading Techniques

The standard technique for reading a CRL involves the reader walking through the components of the CRL and building up a graph of objects where each object more or less corresponds to one of the logical entities on the left-hand side of the schema definition. E.g. an array containing Extension entries or an array of revokedCertificate objects.

The problem with this approach is that it can require a significant amount of memory since every entry on the list is in memory at the same time. However, any information about the CRL can be randomly accessed once the object graph is built which is a convenient property.

Streaming Reading Technique

Imagine working on an assembly line. After a widget has moved past us on the conveyor belt, it’s gone. Likewise, when we are reading the CRL in as a stream we receive data only once and it is inaccessible after that. An input stream is an abstract concept representing the consumption of data as it arrives. Data coming in over the network, for example, arrives in a stream and it is not possible to assume that the data will ever be available again. In reality, when we are working with files on disk, we would have the ability to retrieve data that has already been read once, but we are limited by the constraints of the stream abstraction. See this page for a discussion of I/O in Java.

Streaming Reading Process

This process is focused entirely on retrieving the revokedCertificate entries. It does not return any information stored elsewhere in the tbsCertList nor does it check the validity of the signature. It is possible to do these things but with an increase in complexity. The decision to dispense with signature verification means that this process is inappropriate for a CRL that does not belong to the reader. Reading a CRL issued by another entity without verifying the signature potentially exposes the reader to using a counterfeit CRL. I will discuss alterations to the process that could be made to address these deficiencies.

The reading process has four elemental operations. Each one of these operations can be passed a counter to track the number of bytes that it has consumed from the stream.

  • Reading the tag reads a single byte.
  • Reading the tag number takes the tag byte and extracts the tag number from it by looking at bits five through one. It is important to note that ASN1 does allow for tags that use multiple bytes but the only tags that do so are tags representing data types that are not “universal”, i.e. not already included in the ASN1 specification. The CRL specification, however, does not use any non-universal tags (barring any custom extensions), so we can ignore this complication when discussing the high level mechanics in this document.
  • Reading the length begins by reading a single byte. If the eighth bit is a zero, interpret the other seven bits as the length, if it is one, the other seven bits indicate how many more bytes to read. Read that many bytes and interpret the result as an unsigned integer.
  • Reading the value reads N bytes and stores them into a byte array. N is generally the value of the length we just read.

To only return the revokedCertificates entries, we need to begin by getting to the beginning of the revokedCertificates sequence. The following process is performed when the caller first constructs the streaming reader:

  1. Begin by reading the tag and length of the TLV for the CertificateList sequence. Discard these values. We have now descended into the tbsCertList.
  2. Read the tag and the length of the tbsCertList. We are now at the version.
  3. Read tag, length, and value until we reach a tag number of type UTCTime or GeneralizedTime. Now we are at the thisUpdate. Read the tag, length, and value.
  4. We are now either at nextUpdate or revokedCertificates. Read the tag and read the tag number. If it is of type UTCTime or GeneralizedTime, read the length, data, and then the next tag. Read the length and store it as seqLength (the length of the revokedCertificates sequence).
  5. We are now within the revokedCertificates sequence.

With the preliminary operations complete, we return the instatiated reader and the caller can begin streaming sequences within revokedCertificates using the next method. This method returns one revoked certificate sequence each time it is called.

  1. Read and store the tag. Decrement seqLength by the number of bytes read.
  2. Read and store the length L. Decrement seqLength by the number of bytes read.
  3. Read L bytes and store them as the value V. Decrement seqLength by the number of bytes read.
  4. Reconstruct the revoked certificate sequence by writing the tag, length L encoded back into DER format, and the value V to a byte array.
  5. Return the byte array. This byte array is a valid DER sequence that can then be used as desired. Callers will generally build a sequence object and then look at the userCertificate or revocationDate.

The caller can repeat the next operation as long as the hasNext method returns true. The hasNext operation simply looks to see if seqLength is greater than zero. If so, hasNext returns true. The purpose of hasNext is to prevent trying to read past the end of the tbsCertList.

Earlier I mentioned that this reading process does not return any meta-data information or perform any signature verification. Returning the meta-data information would be straightforward. The meta-data information is generally on the order of a few hundred bytes so retaining it in memory is not problematic. The caller would need to pass in some type of memo object to the streaming reader when the reader is instantiated. As the reader moves through the meta-data items in the tbsCertList during initialization, it would simply populate the memo object with the values. The caller could then look in the memo object once the reader has finished instatiation.

Signature verification is more complex. Since the signature algorithm used is required for the verification process and the algorithm information is towards the end of the CRL, we would have to make two passes over the CRL. The first pass would skip as rapidly as possible to the signatureAlgorithm and then construct a hasher using the same hash algorithm used by the signatureAlgorithm. Then we would need to reopen the CRL as a new stream (so that we start back at the beginning) and perform our normal stream read operations. As we read, we would pass in every byte from the tbsCertList into the hasher. (Hashers have the useful property of being able to accept bytes over time). Once hasNext detects that the reader has reached the end of the tbsCertList, we would ask the hasher to return the hash and encrypt it with the issuer’s public key using the encryption algorithm specified in the signatureAlgorithm. We would then continue to read the stream down to the signatureValue and compare it to the signature we just calculated. At this point, hasNext can raise an exception or set a flag if the signatures do not match.

Standard Manipulation Techniques

As mentioned before, the standard technique for reading involves creating a complete object graph in memory. The standard technique for writing is to manipulate the graph, adding additional revokedCertificates entries for example. Once the manipulations are complete, a writer converts the object graph back into ASN1 as a tbsCertList, calculates the signature for the new CRL, and then inserts the tbsCertList, signature algorithm, and signature into an ASN1 sequence.

The memory requirements for this approach are still high with the additional disadvantage that much of the memory is allocated to objects that CRL manipulation programs are uninterested in. A program that wants to add an entry to the CRL is unlikely to require continual access to the other revokedCertificates entries (aside from a quick check that the entry being inserted isn’t already present). Furthermore, when it comes time to write the CRL, time is wasted converting unchanged items back into ASN1.

The standard technique is analogous to memorizing an entire grocery list and then writing the whole list on another piece of paper merely to add “eggs” to the bottom of the list. This analogy is slightly deceptive, however, because modifying the CRL invalidates the signature and results in changes to the length values in the ASN1 container sequences. Let’s amend our analogy and add the stipulation that the list is written in ink and the paper is cut to the exact length of the list. With these new constraints, copying the list over to another piece of paper is a reasonable and straightforward approach albeit a laborious one.

Streaming Manipulation Techniques

There is another way to modify our hypothetical grocery list. What if we write “eggs” on a strip of paper and glue it on to the bottom of our existing list? This solution is fast and means that we don’t have to reproduce the whole list again, but it is potentially ugly and messy. The streaming technique described below is the equivalent of our pasted together grocery list complete with the same virtues and liabilities.

The Streaming Manipulation Process

At this point, let’s discuss the L in an ASN1 TLV. The L, length, must be unfailingly accurate. If it is inaccurate, other parsers will be unable to decode what we wrote. They will end up reading an incorrect number of bytes for the value portion of the TLV. The parser would then end up trying to read in another TLV at an incorrect position and will invariably fail.

Imagine a script where the character’s name, the T, is followed by the number of lines they will speak, the L, and the lines themselves are the V.

HORATIO: 1 lines
    Hail to your lordship!
HAMLET: 2 lines
    I am glad to see you well:
    Horatio,--or I do forget myself.
HORATIO: 1 line
    The same, my lord, and your poor servant ever.
HAMLET: 2 lines
    Sir, my good friend; I'll change that name with you:
    And what make you from Wittenberg, Horatio? Marcellus?

Now imagine we inadvertently change the first direction to read “3 lines” instead of “1 line”. Our literal-minded actors would speak the following dialog:

<Horatio>: "Hail to your lordship!  Hamlet 2 lines.  I am glad to see you well:"
<Horatio>:

And then our actor playing Horatio stops because while “Horatio” is a valid T, “–or I do forget myself” can’t be interpreted as a number of lines to read (an L).

This analogy illustrates the problem with an incorrect length in a single TLV. However, our TLVs can contain other TLVs by using the ASN1 sequence type much like if our play also had the number of lines specified for each scene and act. If we are adding additional dialog we need to remember to update the number of lines in the scene and the number of lines in the act. Like throwing a stone into a pond, a length change deep within nested structure ripples outward to the containing structures.

A further complication ensues when we realize that the number of bytes used to encode a length is factored in to length of the containing sequence. Let’s say we have a sequence and it contains multiple TLVs. On one of those TLVs, we add a single byte to the value portion. Consequently, we need to increase the length of this TLV by 1. If the current length value is 127, the length value itself would require a single byte. But increasing the length to 128 results in DER requiring two byte (the first bit is reserved in the first length byte). Therefore the length of our container sequence needs to increase by 2! Once for the actual byte we added to the V in our constituent TLV and once for the additional byte now required to encode the L in the same TLV. Accordingly, when we compute the differences in lengths, we also need to account for the difference in bytes required to encode each length in DER. The function used for this accounting is called findHeaderBytesDelta(oldLength, newLength).

Initialization

When creating the writer instance, some initial information is required. We need to know the public and private key to use for signing the CRL. Since the size of the signature depends on the size of the private key, we must do some math to account for a CRL being signed with a different sized key. The length of an RSA signature (the only signature type currently supported by this method) is equal to the length of the private key’s modulus in bytes (see here for more information on RSA). We create an empty byte array sized the same as the signature length and then encode that in DER as a bit string type. We take the length of the resulting dummy signature and store that as newSigLength. We will retain the public key to insert into the AuthorityKeyIdentifier extension that is required by RFC 5280 for version 2 CRLs.

Our modifications can include either additions of revoked certificate entries or deletions of existing entries where the certificate has reached its expiration data and will thus never be valid in any circumstance.

For deletions, during this first pass, the caller must specify the serial numbers that should be removed or a callback function that will be invoked with the value of each existing revokedCertificates entry. If the callback returns true, that entry is marked for deletion.

For additions, the caller must specify the userCertificate, revocationDate, and any extensions they desire (such as a revocation reason). We encode this information in DER to create a revokedCertificates entry and store each entry in a list, newEntries. We total the length of each entry and store it as addedEntriesLength.

The First Pass

In order to create a valid CRL, we need to modify several pieces of information besides just the revokedCertificates entries. Unfortunately, some of this information is stored at the end of the CRL. The solution is to make two passes through the CRL. In the first pass, we record pieces of existing data that we need during the modification process.

The first pass begins by using the streaming read process to examine each revokedCertificates entry. If the entry has a serial number matching one of the serials the caller marked for deletion, we record the length of that entry’s ASN1 sequence. At the end, we total all the lengths to create deletedEntriesLength.

Having reached the end of the revokedCertificates sequence, we are potentially at the crlExtensions. RFC 5280 requires two extensions for version 2 CRLs. We continue the read tag, read length, and read value steps pausing after each value is read to assess the whole TLV. If the TLV has a tag number of 0, we are at the crlExtensions (this 0 tag value is unusual but is described in the RFC). Store the subsequent TLV in its entirety as oldExtensions.

If the TLV has a tag number indicating it is a sequence, we have reached the signatureAlgorithm sequence. Read the sequence TLV and store it as signingAlg. Then read the TLVs within signingAlg. The first sequence item is the most important and corresponds to an OID. We compare the OID we read against a reference list. Each OID corresponds to a particular hasher and signer. Based on the OID, we construct the appropriate hasher and signer.

Ultimately, we will reach the signatureValue TLV. We read that TLV and store the number of bytes as oldSigLength.

The oldExtensions require some adjustments. We need to examine each extension looking for the crlNumber extension. The crlNumber is supposed to increase every time the CRL is regenerated. The extensions are just ASN1 sequences, so we continue the pattern of reading the tag, length, and value to introspect each extension. The first item in the sequence will be an OID (an ASN1 object identifier). We are looking for OID 2.5.29.20 which the RFC has assigned to mean crlNumber. If the item matches, we read the next TLV which is an ASN1 octet string. We interpret this octet string as an integer and add one to it.

The other extension that may need changing is the authorityKeyIdentifier. If, when iterating through the extensions, we find OID 2.5.29.35, then replace the next TLV with the public key passed in during initialization encoded as an AuthorityKeyIdentifier according to RFC specifications.

Store the modified extensions in DER as newExtensions and calculate the entire difference in length using the formula newExtensions.length - oldExtensions.length and store the result as extensionsDelta. Since both newExtensions and oldExtensions are DER encoded, there is no need to run findHeaderBytesDelta on the two since the difference in the DER encoded lengths already accounts for the difference in the header bytes.

The first pass is now complete. We have gathered four pieces of information: the total number of bytes we plan on removing due to deleted entries, the DER encoded new extensions, the hashing and signing algorithms to use, and the number of bytes in the old signature.

The Second Pass

The second pass is where we write the modified CRL to output. We open a new input stream for the CRL and perform the following procedure:

  1. Read the tag and length. Store both these values as topTag and oldTotalLength.
  2. We have moved to the tbsCertList. Read the tag and length and store them as tbsTag and oldTbsLength.
  3. Read the tag, length, and value for the initial entries of the tbsCertList storing in tempOutput as we go.
  4. When we read a tag with a tag number of type GeneralizedTime or UTCTime, we have reached the thisUpdate field. Since we are updating the CRL, we need to update this value.
  5. Read the length and value. Store the value as oldThisUpdate. Based on the tag number construct a new GeneralizedTime or UTCTime with an appropriate time (such as the current time) and encode it to DER. Store the new DER for thisUpdate in tempOutput.
  6. Potentially, we are now at the nextUpdate field. It would be possible to allow callers to specify this value, but instead I elected to set it to the new thisUpdate time plus the difference between the old thisUpdate and the old nextUpdate. Read the tag and store it in tempOuput. If its tag number is a GeneralizedTime or UTCTime, we need to write a new nextUpdate. Read the length and value. Store the value as oldNextUpdate. Compute the time difference between oldThisUpdate and oldNextUpdate. Add that difference to the current time and encode that time in DER as the same type (GeneralizedTime or UTCTime) used originally. Write the new nextUpdate to tempOutput. Read the next tag and store it to tempOutput.
  7. We are now at the revokedCertificates. Read the tag and length. Store the length as oldRevokedCertsLength. Our stream is now at the start of the first revokedCertificates entry.
  8. Pause the reading for a moment to adjust the preceding sequence lengths:
    • newRevokedCertsLength = addedEntriesLength - deletedEntriesLength + oldRevokedCertsLength
    • revokedCertsHeaderBytesDelta = findHeaderBytesDelta(oldRevokedCertsLength, newRevokedCertsLength)
    • tbsCertListLengthDelta = addedEntriesLength - deletedEntriesLength + revokedCertsHeaderBytesDelta + extensionsDelta
    • newTbsLength = oldTbsLength + tbsCertListLengthDelta
    • tbsHeaderBytesDelta = findHeaderBytesDelta(oldTbsLength, newTbsLength)
    • sigLengthDelta = newSigLength - oldSigLength
    • totalLengthDelta = tbsCertListLengthDelta + tbsHeaderBytesDelta + sigLengthDelta
    • newTotalLength = oldTotalLength + totalLengthDelta
  9. Having calculated the length changes for revokedCertificates, tbsCertList and CertificateList, we can begin to write the new DER to the output.
    • Write topTag.
    • DER encode and write newTotalLength.
    • Write tbsTag and send it to the signer. (Every byte of tbsCertList must go to the signer).
    • DER encode newTbsLength. Write the result and send it to the signer.
    • Write the contents of tempOutput and send the them to the signer.
    • DER encode newRevokedCertsLength. Write the result and send it to the signer.
  10. Begin reading the TLVs for each revokedCertificates entry from the stream. At each read, we will decrement oldRevokedCertsLength by the number of bytes we have read. If the entry we read has a serial that we were asked to delete, drop the entire entry. Otherwise, write the TLV to the output and send it to the signer.
  11. Once oldRevokedCertsLength has been reduced to zero, write each entry in newEntries to the output and send each to the signer.
  12. Write newExtensions to the output and send it to the signer.
  13. Write signingAlg to the output.
  14. Ask the signer to generate a signature. Encode that signature as a BIT STRING and write to the output.

The Degenerate Case

There is one case where the streaming process is not appropriate. If the CRL is empty, the revokedCertificates sequence is simply not present in the ASN1. Handling this case leads to difficulties as once we have proceeded through to the nextUpdate, we would have to add a large amount of conditional logic to determine whether we had reached the revokedCertificates or crlExtensions and then handle the situation accordingly. It is probably possible to add this logic, but at the expense of code readability. Instead, I elected to handle an empty CRL as an exceptional case. When we detect that there is no revokedCertificates sequence during the first pass, a flag is set to true. During the second pass, if this flag is true, we abort the streaming process and use the conventional approach of reading into memory, manipulating the object graph, and encoding the graph into DER. This approach performs well enough for small CRLs so long as we are not adding thousands upon thousands of entries.

Certificate Revocation List & Online Certificate Status Protocol

The CRL(Certificate Revocation List) is generated by CertificateRevocationListTask on a regular basis for all the entitlements which have been revoked.

  • The location where the crl is generated can be configured by specifying the property, by default the file is in /var/lib/candlepin/candlepin-crl.crl

    candlepin.crl.file=/etc/candlepin/candlepin-crl.crl
    
  • To change the frequency

    pinsetter.org.candlepin.pinsetter.tasks.CertificateRevocationListTask.schedule = 0 5 * * * ?
    

    in the candlepin.conf file.

You can tweak the frequency at which the crl file is generated by changing DEFAULT_SCHEDULE in CertificateRevocationListTask.

Configuring CRL with ocspd

To verify the integrity of the CRL generated as well as use it in practice using ocspd, please follow the following steps.

  • Install ocspd. On fedora use the following command to install ocspd

        sudo yum install ocspd -y
    
  • Configure ocspd. You may have to customize to fit your needs. After doing so, please move the file to /etc/ocspd/ocspd.conf
  • Start your ocspd daemon using the following command

         ocspd -v -d 9090 -c /etc/ocspd/ocspd.conf
    

    ocspd logs in /var/log/messages. When you try the above command, you should see something similar to the contents of [attachment:log.txt log.txt] file.

  • Query the status of a serial using the following command,

        openssl ocsp -issuer /etc/candlepin/certs/candlepin-ca.crt -serial 0x01 -host localhost:9090 -CAfile /etc/candlepin/certs/candlepin-ca.crt
    

    When you query for some serial which is valid and not present in the crl list, you should see something similar to the text below.

      Response verify OK
      0x01: good
      This Update: Jul  7 18:22:30 2010 GMT
      Next Update: Jul  7 18:41:48 2010 GMT
    

    When you query for a serial which is not valid/revoked.(present in crl file),

      Response verify OK
      0x31: revoked
      This Update: Jul  7 18:22:30 2010 GMT
      Next Update: Jul  7 18:42:03 2010 GMT
      Reason: (UNKNOWN)
      Revocation Time: Jul  7 00:00:00 2011 GMT
    

Reference

Last modified on 29 March 2017