Computer System Security
Useful Functions from the dcrypt Library

Below is a description of some of the functions implemented in the dcrypt library that you may find useful in completing the lab assignments. You will need to include the dcrypt.h header file to access these functions. You may also want to take a look at these sample programs (tst.c, tst_sha1.c) to see some of these functions in action.

Data serialization

• void putint (void *dp, u_int32_t val);
• void puthyper (void *dp, u_int64_t val);
  The putint function puts the 32-bit integer value of val into memory in big-endian order at location dp. dp does not need to be aligned. The bytes stored at dp will be the same on big- and little-endian machines. puthyper is like putint but puts a 64-bit value into 8 bytes of memory.

• u_int32_t getint (const void *dp);
• u_int64_t gethyper (const void *dp);
  The getint and gethyper routines retrieve values stored by putint and puthyper respectively.

• char *armor64 (const void *dp, size_t len);
  Transforms len bytes from the binary string pointed by dp to a longer, base-64, printable ASCII string. You will need to use this to transform random session keys (which could contain zero-bytes) into a NULL-terminated ANSI C string.

• ssize_t dearmor64 (void *out, const char *s);
  Inverts the armor64 function, and return the number of bytes that were placed at out. The return value is negative if the NULL-terminated ANSI C string s is not the output of armor64.

• ssize_t armor64len (const char *s);
  Tries to find an armored string starting at the byte pointed to by s. If some prefix of s represents a valid armor64 string, then the length of such prefix is returned. Otherwise, -1 is returned, indicating that s is not the output of armor64.

• ssize_t dearmor64len (const char *s);
  Tries to find an armored string starting at the byte pointed to by s. If some prefix of s represents a valid armor64 string, then the length of the decoded data that would result by "dearmoring" s is returned. Otherwise, -1 is returned, indicating that s is not the output of armor64.

Pseudo-Random Number Generation Functions

The libraries you are using contain a cryptographic pseudo-random number generator, whose state is kept in a global 16-byte array called prng_state. Before using the random number generator, you must initialize it.

• void prng_seed (void *buf, size_t len);
  This function initializes the state of the random number generator using len bytes from buf as seed. Providing a good seed may be a difficult task; some Operating Systems (including FreeBSD, OpenBSD and most Linux distributions) provide you with a source of randomness under /dev/random (or one of its variants: /dev/srandom, /dev/urandom, etc.). If a random device is available, you should read (at least) 128 bits from it and use it as a seed; otherwise, as a very rough approximation, you could supply some information about your local machine (e.g., time of the day, PID/GID value) that is difficult to predict.

• void prng_getbytes (void *buf, size_t len);
  Writes len pseudo-random bytes to memory at location buf.

• u_int32_t prng_getword ();
  
• u_int64_t prng_gethyper ();
  These functions return a single pseudo-random 32- or 64-bit integer, respectively.

Symmetric-Key Encryption Functions

For actually encrypting and decrypting file data, you will use the Rijndael [FIPS-197] block cipher (also called AES---Advanced Encryption Standard). Rijndael is a 128-bit block cipher. It supports two operations--encryption, and decryption. Encryption transforms 16 bytes (128 bits) of plaintext data into 16 bytes of ciphertext data using a secret key. Someone who does not know the secret key cannot recover the plaintext from the ciphertext. The decryption algorithm, given knowledge of the secret key, transforms ciphertext into plaintext.

The libraries you are using define a struct called aes_ctx that you should use to hold secret keys for AES. You should manipulate your AES secret keys with the following functions:

• void aes_setkey (aes_ctx *aes, const void *key, u_int len);
  This sets the secret encryption key using len bytes fro the buffer key. The key must be 16-, 24-, or 32-long.

• void aes_encrypt (const aes_ctx *aes, void *buf, const void *ibuf);
aes_encrypt transforms 16 bytes of plaintext data at ibuf into 16 bytes of ciphertext data which it writes to buf. It uses the secret key previously stored within aes using the aes_setkey function.

• void aes_decrypt (const aes_ctx *aes, void *buf, const void *ibuf);
aes_decrypt decrypts 16 bytes, inverting the aes_encrypt function.

• void aes_clrkey (aes_ctx *aes);
  This clears the content of aes, thus wiping out the secret encryption key that was previosly stored there.
  Once you are done with a secret key, you should always wipe out the memory location where the secret key was stored! The Operating System safeguards your memory from external processes only while that piece of memory is assigned to your program: once your program exits, the memory location used to store your precious secret key could be assigned to another process, jeopardizing the security of your application.

Public-Key Encryption Functions

If you only used symmetric-key cryptography, you would need to exchange a secret key with all the friends with whom you want to have confidential communication. With Public-Key Cryptography, instead, you can give all your friend the same public key, and they will be able to send you encrypted content that only you can recover. Similarly, you only need to know your friend's public key to create a ciphertext that only he/she will be able to decrypt. The libraries you are going to use contain an implementation of two very well-known Public-Key Encryption schemes: Rabin [Wil80] and ElGamal [ElG86] To complete the assignment, you don't need to know anything about how these schemes work, except for the interface that they provide, which is described below:

• dckey *dckeygen (const char *type, size_t k, const char *extra);
  Returns a pointer to a struct that holds a new private key/public key pair, of the type specified by type. The value of type should be one of the constants defined in dcrypt.h, namely DC_ELGAMAL or DC_RABIN.

  The cryptographic strength of the key pair generated can be tuned using the parameter k: a value of at least 1024 is recommended to get a reasonable level of security.

  The value of extra should be either NULL, or an ASCII string representing information about the parameters that should be used in generating the private key/public key pair.
  For this assignment, you can always supply a value of NULL.

  Notice that the dckeygen function internally makes use of the pseudo-random number generator provided by libdcrypt: therefore, you shouldn't call dckeygen before initializing the pseudo-random number generator with prng_seed.

• char *dcexport_pub (const dckey *key);
• char *dcexport_priv (const dckey *key);
  The dcexport_pub and dcexport_priv functions return a base-64, printable, NULL-terminated ANSI C string representing the public key or private key stored within key, respectively.

• dckey *dcimport_pub (const char *asc);
• dckey *dcimport_priv (const char *asc);
  The dcimport_pub and dcimport_priv functions retrieve the dckey that was previously exported into the ASCII string asc using dcexport_pub or dcexport_priv, respectively.

  If asc is not of the form expected, dcimport_pub and dcimport_priv return NULL.

• char *dcencrypt (const dckey *key, const char *msg);
dcencrypt uses the public key contained in key to transform the plaintext data contained in the NULL-terminated ANSI C string msg into a NULL-terminated ANSI C ciphertext string, which is returned as output.

  The encryption algorithm used is CCA-secure.

  Since dcencrypt expects the plaintext to be a NULL-terminated ANSI C string, you should use the armor64 function if you need to encrypt an arbitrary bit string (such as a secret key for AES).

• char *dcdecrypt (const dckey *key, const char *cmsg);
aes_decrypt decrypts the ciphertext contained in the NULL-terminated ANSI C string cmsg, inverting the dcencrypt function.

• void dcfree (dckey *key);
  This clears the content of key. Once you are done with a private key, you should always wipe out the memory location where it was stored!

• int dcispriv (const dckey *key);
  Returns a non-zero value if the key contained in the key is a private key; otherwise it returns 0.

Cryptographic Hash Functions

The SHA-1 [FIPS-180-1] hash function hashes an arbitrary-length input (up to 2^64 bytes) to a 20-byte output. SHA-1 is known as a cryptographic hash function. While nothing has been formally proven about the function, it is generally assumed that SHA-1 is one-way and collision-resistant. These properties are defined as follows:

• A one-way function is a function that is cheap to compute, but computationally intractable to invert. For example, Unix uses a one-way hash function to hash users' passwords, and stores password hashes rather than actual passwords. When a user logs in, the user types a password, Unix hashes this password and compares the new hash to the one stored. If they match, the login is successful.

  For someone who steals the file of password hashes, there is no known way of recovering passwords more efficient than guessing passwords and verifying the guesses. (Of course, the fact that users often choose easily-guessed passwords is a problem.)

• A collision-resistant hash function is one for which it is computationally intractable to find any two inputs that yield the same output. In the case of SHA-1, there are, of course, a huge number of collisions. One can see this by a simple counting argument--there are 2^55,340,232,221,128,654,848 possible inputs to the function and only 2^160 possible output values. Nonetheless, no one has ever succeeded in finding two inputs producing the same output--even cryptographers specifically analyzing the algorithm for this purpose.

  Collision-resistant functions have many uses, stemming from the fact that the short output value effectively uniquely specifies an arbitrary-length input. One cannot recover the input from the output, but given the input, one can verify that it does, indeed, match the output. One might, for instance, implement a web cache in which contents is indexed by a SHA-1 hash of the URL. Having fixed-length names for stored content would simplify the implementation.

• void sha1_hash (void *digest, const void *buf, size_t len);
  Hashes len bytes of data at buf, and places the resulting 20 bytes at digest.

Sometimes the input that you want to hash is so long that it is inconvenient to store it entirely in memory before being able to hash it. This is the case for example when hashing the entire content of a file into a short digest.
For this reason, the libraries you are using allow you to process a long input "one chunk at a time." To do that, you should use a struct called sha1_ctx, which will store the "partial digest" as you keep providing new input to be hashed. You should manipulate sha1_ctx structs with the following functions:

• void sha1_init (sha1_ctx *sc);
  Initializes the sha1_ctx struct that will contain the partial hash.

• void sha1_update (sha1_ctx *sc, const void *data, size_t len);
  Adds len bytes at data to the input being hashed, but does not produce a result. Thus, one can hash a large amount of data without having it all in memory, by calling sha1_update on one chunk at a time.

• void sha1_final (sha1_ctx *sc, void *digest);
  Produces the final hash, and places the resulting 20 bytes at digest.

Message Authentication Codes (MACs)

Message Authentication Codes (MACs) are a symmetric-key primitive allowing you to check the integrity of the information to which the MAC is applied. Recall that encryption does not guarantee integrity! The fact that you were able to decrypt a ciphertext is not enough to be sure that nobody tampered with its content. For integrity, you should always append a MAC to the content.

You use MACs as follows. Let's say that you want to store a file on your file-server, but you are afraid that its content will be changed behind your back. Then, you use a secret key to "mac" the file, and store the resulting MAC along with the file. Now, when you check back with your file-server and retrieve your file, you will also retrieve the MAC that you appended. Then, you will use the secret key to compute the MAC again, and if the MAC you just computed is the same as the value that you retrieved from the file-server, then you are sure that nobody touched your file. This is because, if somebody had changed the file, then they should have computed the corresponding MAC in order to fool you. However, secure MACs are concocted such that, without knowing the secret key, it is computationally intractable to compute the right MAC, even after having seen a lot of valid (message, MAC) pairs.

The Keyed-Hash Message Authentication Code (HMAC) [FIPS-198a] is a secure Message Authentication Code based on the use of any cryptographic hash function, like SHA-1. The libraries you are using contain an implementation of HMAC, instantiated with the SHA-1 cryptographic hash function.

• void hmac_sha1 (const char *key, size_t keylen, void *out, const void *data, size_t dlen);
  Computes the HMAC over len bytes of data at buf using the key key, and places the resulting 20 bytes at out.

Similarly to what discussed for the case of SHA-1, the libraries you are using allow you to process a long input "one chunk at a time."

• void hmac_sha1_init (const char *key, size_t keylen, sha1_ctx *sc);
  Initializes the sha1_ctx struct that will contain the partial HMAC under the key key.

• void hmac_sha1_update (sha1_ctx *sc, const void *data, size_t len);
  Adds len bytes from data to the input being hmac'ed, but does not produce a result. Thus, one can hmac a large amount of data without having it all in memory, by calling hmac_sha1_update on one chunk at a time.
  Notice that the key is not needed when adding chunks.

• void hmac_sha1_final (const char *key, size_t keylen, sha1_ctx *sc, void *out);
  Produces the final HMAC, and places the resulting 20 bytes at out.
  Notice that the result is undefined if the key used in hmac_sha1_final is different from the one initially used in hmac_sha1_init.