RSA Security 5.2.2 manual RSA Algorithm, 20Input Limits for RSA Pkcs Encryption

System Considerations In Crypto-C

Crypto-C offers padding for the symmetric block-encryption algorithms, which have no restrictions on the total input length. Padding means that the total length of the encrypted data can be as many as eight bytes more than the total length of the input.

Note: For algorithm info types that supply padding, Crypto-C will pad even if the input is a multiple of the block size. This way, when decrypting, Crypto-C knows that the last byte is guaranteed to be a pad byte. For AIs that use PKCS #5 padding, the last byte, when decrypted, will be a number: the number of pad bytes Crypto-C should strip.

The RSA Algorithm

The second common input constraint is the RSA algorithm. Recall that this algorithm uses modular math.

Input constraints

The following input restrictions apply:

•Whenever modular math is used a calculation, the values passed must be less than the RSA modulus n. For example, if the modulus is 55, the input must be from zero to 54; the number 57 is invalid.

•For RSA encryption that is PKCS v1.5-compatible, the input to encryption or decryption must be no more than k – 11 bytes long, where k is the modulus length in bytes. For example, with a 768-bit modulus, the input can be no more than 85, or 96 – 11, bytes. This is because the padding scheme needs at least an 11-byte area to work. The output will be the same size as the modulus.

For the most common key sizes, Table 4-20, which follows, shows the maximum number of bytes you can encrypt using the RSA algorithm with PKCS padding.

Table 4-20Input Limits for RSA PKCS Encryption

	Modulus Length (k) in	Maximum Number of Bytes
Key Size in Bits	Bytes	That Can Be Encrypted (k-11)
512	64	53
768	96	85
1024	128	117
2048	256	245

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