/// <summary>
/// Generate the "well-known constant"
/// [RFC 3961, Page 15]
/// the "well-known constant" used for the DK function is the key usage number,
/// expressed as four octets in big-endian order, followed by one octet indicated below:
/// Kc = DK(base-key, usage | 0x99);
/// Ke = DK(base-key, usage | 0xAA);
/// Ki = DK(base-key, usage | 0x55);
/// </summary>
/// <param name="usage">key usage number</param>
/// <param name="derivedKeyType">the derived key type</param>
/// <returns>the "well-known constant"</returns>
private static byte[] GetWellKnownConstant(int usage, DerivedKeyType derivedKeyType)
{
// the "well-known constant" contains 5 bytes
byte[] wellKnownConstant = new byte[5];
// the first 4 bytes = usage number in big endian
byte[] usageBytes = BitConverter.GetBytes(usage);
Array.Reverse(usageBytes);
usageBytes.CopyTo(wellKnownConstant, 0);
// the 5th byte = the derivedKeyType
wellKnownConstant[4] = (byte)derivedKeyType;
return(wellKnownConstant);
}
/// <summary>
/// Generate a derived key from base key
/// [RFC 3961 Section 5.1 A Key Derivation Function]
/// </summary>
/// <param name="baseKey">the base key</param>
/// <param name="usage">key usage</param>
/// <param name="derivedKeyType">the derived key type</param>
/// <param name="aesKeyType">AES key type which decides key size</param>
/// <returns>the derived key in bytes</returns>
internal static byte[] MakeDerivedKey(
byte[] baseKey,
int usage,
DerivedKeyType derivedKeyType,
AesKeyType aesKeyType)
{
if (null == baseKey)
{
throw new ArgumentNullException("baseKey");
}
// generate the well-known constant
byte[] wellKnownConstant = GetWellKnownConstant(usage, derivedKeyType);
// generate the derived key
return(DK(baseKey, wellKnownConstant, aesKeyType));
}
/// <summary>
/// Generate the "well-known constant"
/// [RFC 3961, Page 15]
/// the "well-known constant" used for the DK function is the key usage number,
/// expressed as four octets in big-endian order, followed by one octet indicated below:
/// Kc = DK(base-key, usage | 0x99);
/// Ke = DK(base-key, usage | 0xAA);
/// Ki = DK(base-key, usage | 0x55);
/// </summary>
/// <param name="usage">key usage number</param>
/// <param name="derivedKeyType">the derived key type</param>
/// <returns>the "well-known constant"</returns>
private static byte[] GetWellKnownConstant(int usage, DerivedKeyType derivedKeyType)
{
// the "well-known constant" contains 5 bytes
byte[] wellKnownConstant = new byte[5];
// the first 4 bytes = usage number in big endian
byte[] usageBytes = BitConverter.GetBytes(usage);
Array.Reverse(usageBytes);
usageBytes.CopyTo(wellKnownConstant, 0);
// the 5th byte = the derivedKeyType
wellKnownConstant[4] = (byte)derivedKeyType;
return wellKnownConstant;
}
/// <summary>
/// Generate a derived key from base key
/// [RFC 3961 Section 5.1 A Key Derivation Function]
/// </summary>
/// <param name="baseKey">the base key</param>
/// <param name="usage">key usage</param>
/// <param name="derivedKeyType">the derived key type</param>
/// <param name="aesKeyType">AES key type which decides key size</param>
/// <returns>the derived key in bytes</returns>
internal static byte[] MakeDerivedKey(
byte[] baseKey,
int usage,
DerivedKeyType derivedKeyType,
AesKeyType aesKeyType)
{
if (null == baseKey)
{
throw new ArgumentNullException("baseKey");
}
// generate the well-known constant
byte[] wellKnownConstant = GetWellKnownConstant(usage, derivedKeyType);
// generate the derived key
return DK(baseKey, wellKnownConstant, aesKeyType);
}
