Latest version of Digital Signature Standard expands key sizes, adds security

A new version of the federal standard for digital signatures increases the allowable key sizes for the approved digital signature algorithm and adds requirements for using an approved elliptic curve algorithm.

The changes are included in the final version of Federal Information Processing Standard 186-3, the Digital Signature Standard, which the secretary of the Commerce Department has approved. It is the third iteration of a standard originally published in 1994. Since then, advances in computing power have made it easier to crack the original 512- and 1,024-bit keys specified in the standard.

“With advances in technology, it is prudent to consider larger key sizes,” according to the statement Commerce published in the Federal Register announcing its approval of the standard. “FIPS 186-3 allows the use of 1,024-, 2,048- and 3,072-bit keys.”

The government plans to move away from key size alone as a way to ensure the security and reliability of digital signatures and will take advantage of the greater security of elliptic curve cryptography.

Government cryptographic tools must comply with Federal Information Processing Standards written by the National Institute of Standards and Technology. NIST released the first draft of Version 3 of the standard for public comment in 2006 and published a final draft for comment in November 2008.

In response to public comments on the standard, NIST said, “The length of the larger keys has a huge impact on communications and storage requirements. The strategy of the U.S. government is to transition to elliptic curve algorithms in order to reduce the key sizes.”

Such algorithms can be mathematically shown to be more difficult to crack than traditional algorithms with larger key sizes.

Digital signatures use cryptographic algorithms to create a code that is tied mathematically to the message being signed. That code can be used to verify electronic documents in much the same way a written signature is used with paper documents.

“Digital signatures are used to detect unauthorized modifications to data and to authenticate the identity of the signatory,” the NIST standards document states. “In addition, the recipient of signed data can use a digital signature in proving to a third party that the signature was, in fact, generated by the claimed signatory. This is known as non-repudiation, since the signatory cannot repudiate the signature at a later time.”

FIPS 186 defines methods for digital-signature generation that can be used to protect electronic data and verify and validate those signatures. The standard originally specified only the Digital Signature Algorithm (DSA) as an approved tool. Later revisions added versions of Elliptic Curve DSA (ECDSA) and the Rivest-Shamir-Adleman (RSA) algorithm.

“Although all three of these algorithms were approved in FIPS 186-2, FIPS 186-3 increases the key sizes allowed for DSA, provides additional requirements for the use of RSA and ECDSA, and includes requirements for obtaining the assurances necessary for valid digital signatures,” the Commerce announcement states.

Another change is that specifications for random number generators are not included in FIPS 186-3. The standard refers instead to NIST special publications for requirements in obtaining random numbers. Methods for random number generation specified in FIPS 186-2 are being phased out, and the only approved methods will be those in SP 800-90.

That approach is part of a trend toward moving technical specifications out of standards documents and referring instead to the NIST special publication series, which can be updated more quickly as technology changes. Methods for ensuring the validity of digital signatures are outlined in NIST SP 800-89, “Recommendation for Obtaining Assurances for Digital Signature Applications.”

The new standard will be available at csrc.nist.gov/publications/PubsFIPS.html.

About the Author

William Jackson is freelance writer and the author of the CyberEye blog.

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