Overview of crypto standards
Zdeněk Říha
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Hash functions
oMD5 (128 bit output) – defined v RFC 1321
oRIPEMD-128/RIPEMD-160 in ISO/IEC 10118-3
oBLAKE2b, BLAKE2s defined in RFC 7693.
o
o
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Symmetric crypto
lModes of operation (FIPS 81)
lECB (Electronic Code Book)
lCBC (Ciper Block Chaining)
lCFB (Cipher Feedback Mode)
lOFB (Output Feedback Mode)
lNewer modes of operation
lCTR (Counter Mode) [FIPS SP 800-38A]
l CMAC [FIPS SP 800-38B], CCM [FIPS SP 800-38C], GCM [FIPS SP 800-38D], XTS-AES [FIPS SP 800-38E]
lOther in FIPS SP 800-38F, FIPS SP 800-38G
l
See:
https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation
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Padding
lISO 9797 method 1 padded with values 0x00
lto remove the padding the length of the original message is needed
lISO 9797 method 2 (ISO 7816-4, EMV’96) – first the value 0x80 is added, then bytes of 0x00 are
added
lPS = ‘80 00‘, if 2 bytes are needed
lPS = ‘80 00 00 00 00 00 00 00‘, if 0 bytes are needed (full block added)
l• PKCS#5 – the padding string is made from value n-(||M|| mod n)
lfor (3)DES n=8, AES n=16
le.g. PS = 02 02 - if 2 bytes are needed
le.g. PS = 08 08 08 08 08 08 08 08 – if 0 bytes are needed and n=8 (3DES)
l
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Symmetric crypto
lDES – defined in FIPS PUB 46 (-1 a -2)
lkey 56 bits, block 64 bits
l3DES – defined in FIPS PUB 46-3
lkey either 112 or 168 bits, block 64 bits
lAES – (Rijndael), defined v FIPS PUB 197
l key 128, 192 or 256 bits, block 128 bits
l
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Asymmetric crypto
lCertificates X.509
lITU-T, ISO/IEC, RFC
lDER / PEM
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PKCS
lPKCS#1 – defines RSA encryption
lPKCS#3 – defines Diffie-Hellman protocol
lPKCS#5 – symmetric encryption based on a password
lPKCS#7 – format for digital signatures and asymmetric encryption
lPKCS#8 – defines the private key format
lPKCS#10 – defines format for certificate requests
lPKCS#11 – API for communication with cryptographic tokens
lPKCS#12 – format for storing private keys including public key certificates, all protected by a
password
lPKCS#13 – defines encryption based on elliptic curves
lPKCS#15 – defines cryptographic token information format
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RSA Padding
lE.g. RSA 2048 bits
lModulus n is 2048 bits, public exponent e usually small
lMessage m is 2048 bits in total, usual hash functions provide hashes much shorter. Therefore we
need padding.
lBTW No padding needed for DSA and ECDSA
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RSA Padding algorithms
lANSIX 9.31
l6b bb … bb ba || Hash(M) || 3x cc
(where x=3 for sha1, x=1 for ripemd160)
lPKCS#1 v1.5
l00 01 ff … ff 00 || HashAlgID || Hash(M)
lPSS
l00 || H  || G(H) Å [salt || 00 … 00] (where H = Hash(salt, M), salt is random, and G is a mask
generation function)
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Assignments
l
1.Write a program (in any programming language) that will prepare a padded block for RSA signature
with PKCS#1 v1.5 padding. Input is a file and RSA key size; output is the padded octet string
(print it in hex). Use SHA-256 as the hash function. Do not use crypto library for the padding
itself [5 points].
2.Write a program that will generate 2048 bit DH parameters in DER format. Use any cryptolibrary
and any programming language. Recommendation: Openssl & C & functions DH_new,
DH_generate_parameters_ex, i2d_DHparams_bio. [5 points].
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Assignment 1
lPKCS#1 v1.5 padding
lWe open the PKCS#1 v2.2 document :-)
lAlso available as RFC 8017
lWe find the relevant section
l9.2 EMSA-PKCS1-v1_5
lEMSA-PKCS1-v1_5-ENCODE (M, emLen)
lInput: Message + length of padded result (key size)
lOutput: EM (the padded results) to be signed
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Assignment 1
lAs we can see in step 5 the result is:
l
l
lwhere PS is composed of 0xff bytes to fit the size
land T is DER encoded structure containing the hash algorithm and hash itself:
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Assignment 1
lThe authors of PKCS#1 are very nice and provide a help for common hash algorithms
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l
l
l
lwhere H is the hash (32 bytes for SHA-256)
lPrint the EM in hex
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Assignment 2
lRead all PKCS#3 standard
l8 pages including introduction, history, …
lAssignment:
lWrite a program that will generate 2048 bit DH parameters in DER format.
l
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Assignment 2
lProgramming language
lUse any cryptolibrary and any programming language.
lRecommendation: Openssl & C & functions DH_new, DH_generate_parameters_ex, i2d_DHparams_bio
lTry “man dh”
lVerify results:
l“openssl asn1parse -inform DER -in yourfile.der”
l“openssl dhparam -inform DER -in yourfile.der -noout -text”
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Good luck
l
lGood luck and good fun while reading the standards
l
lEmail: zriha@fi.muni.cz
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