System Considerations In Crypto-C
126 RSA BSAFE Crypto-C Developer’s Guide
Input and Output
Some of the AI entries in the Reference Manual include the categories “Input
Constraints” and “Output Considerations”:
• Input constraints generally describe the input requirements of the algorithm
specified by the AI.
• Output considerations warn you that there may be more (or fewer) output bytes
than input bytes.
Two algorithm types that typically have input constraints or output considerations
are symmetric block algorithms and the RSA algorithm.
Symmetric Block Algorithms
Symmetric block algorithms may have both input constraints and output
considerations.
Input constraints
• In symmetric block-encryption algorithms, the total number of input bytes must
be a multiple of the block size. That does not mean the input to each call to an
Update function must be a multiple of the block size, just the total.
For instance, with the RC2 algorithm, the block size is eight bytes. You can pass 23
bytes in the first call to Update, then 18, then 7, for a total of 48.
Output considerations
• For a symmetric block-encryption algorithm, the output from each Update call
may be different from the input size.
In the previous example, the RC2 algorithm was able to process 16 of the first 23
bytes but saved 7 in a buffer. The input was 23, but the output was 16. During the
second call to Update, Crypto-C had the 18 new input bytes plus the old 7, or 25
bytes to work with. It could process 24 (and save 1). Hence, the input was 18, but
the output was 24 bytes long. The last 7 input bytes combined with the saved 1
byte make up the final 8-byte block. It is important to allow for this difference in
length between output and input in your application.
• In addition to the difference in size during Updates, the overall data size can
differ between input and output.
