I saw that there are three RSA encryption versions, as follows:

  • RSA encryption version 1.5 (RFC 2313)
  • RSA cryptography specification version 2.0 (RFC 2347)
  • RSA cryptography specification version 2.1 (RFC 3447)

Why is there no RFC for RSA version 1.0?


1 Answer 1


RSA is and was specified by the PKCS#1 specifications of RSA laboratories. PKCS are the "Public Key Cryptography Standards" by RSA Laboratories, now part of EMC2. The RSA PKCS#1 v1.5 is the lowest publicly released version of RSA by RSA labs that can currently be downloaded.

Version 1.0 to 1.4 are working drafts as specified in the PKCS#1 documents themselves and at least the inital PKCS#1 RFC 2313 as well:

  • Versions 1.0–1.3 were distributed to participants in RSA Data Security, Inc.'s Public-Key Cryptography Standards meetings in February and March 1991.
  • Version 1.4 was part of the June 3, 1991 initial public release of PKCS. Version 1.4 was published as NIST/OSI Implementors' Workshop document SEC-SIG-91-18.

Although the RSA standards have been defined by RSA Laboratories they are likely to have been influenced by the participants to the meetings and workshops.

The RFC's are simply a reiteration of the PKCS#1 standards that have undergone public scrutiny. They are not in control by a single private party. So the standard may be identical, but it is at least open. In general the RFC's just keep to the PKCS#1 specifications, but at least in principle follow up documents may deviate from the specifications by RSA.

  • $\begingroup$ PKCS#1 v1.5 ends with a Revision history, which explains what version 1.0-1.3 and 1.4 have been, and the differences since 1.4 $\endgroup$
    – fgrieu
    Nov 12, 2015 at 14:20
  • $\begingroup$ @fgrieu Thanks! your answer really help me! So happy to learn it! Also, I found PKCS#1 V2.2 was updated in 2012( rsa.com/rsalabs/pkcs/files/…). $\endgroup$
    – Matt Elson
    Nov 12, 2015 at 15:39
  • $\begingroup$ @MattElson 2.2 is a very minor change, it only adds SHA-224, SHA-512/256 and SHA-512/384 to the mix of hashing algorithms. You can see this from the changelog at the bottom. $\endgroup$
    – Maarten Bodewes
    Nov 12, 2015 at 23:31
  • $\begingroup$ Sorry everybody, you may have to input those URL's into the wayback archive, because the current owners of RSA have not deemed it necessary to keep them around. $\endgroup$
    – Maarten Bodewes
    Jul 28, 2021 at 13:11

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