Chapter 4 Using Crypto-C 131
System Considerations In Crypto-C
•Total: 484 bytes
In addition, when the most significant bit of the most significant byte of a value is set,
DER calls for a prepended 0 byte, so that it is not interpreted as a negative 2’s
complement number.
For example, converting the decimal number 3,260,571,825 into hex yields
0xC25860B1. As a byte string, it would be:
C2 58 60 B1
which is four bytes long. But is that a negative or positive number? Is the sign bit set,
or is this an unsigned value? To avoid confusion, we prepend a 0 byte, as follows:
00 C2 58 60 B1
Our string is now five bytes long.
For a 768-bit key pair, the most significant bit of the most significant byte of the
modulus and both primes should always be set. So three of the private key’s values
will have a prepended 0 byte. This increases the total key size to 487 bytes. Sometimes
the most significant bit of the most significant byte of the private exponent, prime
exponents and CRT coefficient will be set, sometimes not. So the total bytes could be
as many as 491.
Note: If the public exponent is F4 (01 00 01), do not prepend a 0 byte to that value.
All of this means that when you generate your RSA key pair, you do not know in
advance how big it is going to be when you store it in DER format. You know the
approximate size, but not the exact length.
Crypto-C has the tools to let you know the exact length of your encoded key. When
you call B_GetKeyInfo with a KI_* that specifies that DER-encoded data should be
output, you pass the address of a pointer nto an ITEM structure. Crypto-C drops off a
pointer at that address. If you go to the address indicated by the pointer, you will find
the key information, which includes the key’s length in the len field of the ITEM. Use
that value to find the exact length of your key.