The document introduces CAO, a domain specific language for developing cryptographic software. CAO aims to generate efficient and secure implementations from high-level cryptographic specifications. It has types for booleans, integers, bit strings, vectors and matrices. A bubble sort example demonstrates the language. The document also discusses transforming CAO code, platform specification, using generated code, and protecting against side-channel attacks. Future work includes improving code efficiency and supporting more platforms/countermeasures.

1. Compiling CAO: from Cryptographic
Specifications to C Implementations
Manuel Barbosa David Castro Paulo Silva
HASLab/INESC TEC
Universidade do Minho
Braga, Portugal
April 8, 2014
Grenoble

2. Motivation
〉 Developing cryptographic software is challenging
〉 Performance is usually critical
〉 Many implementations are done directly in assembly
〉 Aggressive optimizations must not change the semantics
〉 Error prone and time consuming
Manuel Barbosa, David Castro, Paulo Silva 1/14

3. CAO Language
〉 Started in the CACE project (FP7) in collaboration with Univ. Bristol
〉 Domain specific language for core cryptographic components
〉 Hash functions, authentication algorithms, signatures, . . .
〉 High level features closer to standards
〉 Supported by a tool chain to assist development
Manuel Barbosa, David Castro, Paulo Silva 2/14

4. CAO Language
〉 Main design goals:
〉 Flexible and configurable for a wide range of platforms (machine
architecture + operating system + compiler + extra libraries)
〉 Incorporate domain specific optimizations early in the compilation
process
〉 Oriented to the implementation of cryptographic APIs
Manuel Barbosa, David Castro, Paulo Silva 3/14

5. CAO Features
〉 Call by value semantics
〉 No input/output support
〉 No language construct to dynamically allocate memory
〉 Highly expressive native types and operators
Manuel Barbosa, David Castro, Paulo Silva 4/14

6. CAO Types
〉 Booleans
def b1 : bool;
def b2 : bool := true;
Manuel Barbosa, David Castro, Paulo Silva 5/14

7. CAO Types
〉 Booleans
def b1 : bool;
def b2 : bool := true;
〉 Integers (arbitrary precision)
def i1 : int;
def i2 : int := 10;
Manuel Barbosa, David Castro, Paulo Silva 5/14

8. CAO Types
〉 Booleans
def b1 : bool;
def b2 : bool := true;
〉 Integers (arbitrary precision)
def i1 : int;
def i2 : int := 10;
〉 Machine integers
def ri1 : register int;
def ri2 : register int := 1;
Manuel Barbosa, David Castro, Paulo Silva 5/14

9. CAO Types
〉 Booleans
def b1 : bool;
def b2 : bool := true;
〉 Integers (arbitrary precision)
def i1 : int;
def i2 : int := 10;
〉 Machine integers
def ri1 : register int;
def ri2 : register int := 1;
〉 Bit strings
def ubs1 : unsigned bits [32];
def ubs2 : unsigned bits [4] := 0b0101;
def sbs1 : signed bits [16];
def sbs2 : signed bits [8] := 1b01010010;
Manuel Barbosa, David Castro, Paulo Silva 5/14

10. CAO Types (cont.)
〉 Rings or fields defined by an integer
def mo1 : mod [5];
def mo2 : mod [2] := [1];
Manuel Barbosa, David Castro, Paulo Silva 6/14

11. CAO Types (cont.)
〉 Rings or fields defined by an integer
def mo1 : mod [5];
def mo2 : mod [2] := [1];
〉 Extension fields defined by a type and a polynomial
def mp1 : mod[ mod [2] <X> / X**7 + X**3 + 1 ];
def mp2 : mod[ mod [11] <Y> / Y**2 + 1 ] := [5*Y + 2] * [7*Y+1];
Manuel Barbosa, David Castro, Paulo Silva 6/14

12. CAO Types (cont.)
〉 Rings or fields defined by an integer
def mo1 : mod [5];
def mo2 : mod [2] := [1];
〉 Extension fields defined by a type and a polynomial
def mp1 : mod[ mod [2] <X> / X**7 + X**3 + 1 ];
def mp2 : mod[ mod [11] <Y> / Y**2 + 1 ] := [5*Y + 2] * [7*Y+1];
〉 Vectors
def v1 : vector [10] of register int;
def v2 : vector [4] of unsigned bits [2] := {
0b00 , 0b01 , 0b10 , 0b11 };
Manuel Barbosa, David Castro, Paulo Silva 6/14

13. CAO Types (cont.)
〉 Rings or fields defined by an integer
def mo1 : mod [5];
def mo2 : mod [2] := [1];
〉 Extension fields defined by a type and a polynomial
def mp1 : mod[ mod [2] <X> / X**7 + X**3 + 1 ];
def mp2 : mod[ mod [11] <Y> / Y**2 + 1 ] := [5*Y + 2] * [7*Y+1];
〉 Vectors
def v1 : vector [10] of register int;
def v2 : vector [4] of unsigned bits [2] := {
0b00 , 0b01 , 0b10 , 0b11 };
〉 Matrices
def m1 : matrix [2, 3] of int;
def m2 : matrix [2, 2] of mod [2] := {
[1], [0], [0], [1] };
Manuel Barbosa, David Castro, Paulo Silva 6/14

14. Simple Example: Bubble Sort
typedef int_vector := vector [10] of int;
def bubble_sort (v : int_vector) : int_vector {
def temp : int;
seq i := 8 to 0 by -1 {
seq j := 0 to i {
if (v[j] > v[j+1]) {
temp := v[j];
v[j] := v[j+1];
v[j+1] := temp;
}
}
}
return v;
}
Manuel Barbosa, David Castro, Paulo Silva 7/14

15. Simple Example: Bubble Sort
def bubble_sort (const n : register int {1 < n}, v : vector[n] of int)
: vector[n] of int {
def temp : int;
seq i := n - 2 to 0 by -1 {
seq j := 0 to i {
if (v[j] > v[j+1]) {
temp := v[j];
v[j] := v[j+1];
v[j+1] := temp;
}
}
}
return v;
}
Manuel Barbosa, David Castro, Paulo Silva 8/14

16. Complete Algorithm: SHA1
〉 (example sha1.cao)
Manuel Barbosa, David Castro, Paulo Silva 9/14

17. Exploring Intermediate CAO Code
〉 Source to source transformations
〉 (demo)
Manuel Barbosa, David Castro, Paulo Silva 10/14

18. Platform Specification
〉 (demo)
Manuel Barbosa, David Castro, Paulo Silva 11/14

19. Using the Generated Code
〉 (demo)
Manuel Barbosa, David Castro, Paulo Silva 12/14

20. Protection Against Side-channel Attacks
〉 Popular countermeasure against side-channel attacks
〉 Indistinguishable functions:
〉 Vulnerable functions execute the same sequence of native CAO
operations
〉 (demo)
Manuel Barbosa, David Castro, Paulo Silva 13/14

21. Conclusions
〉 The code of the compiler is reasonably stable
〉 The source code is available from the Hackage repository:
http://hackage.haskell.org/package/cao
〉 Future work:
〉 Improve efficiency of the generated code (more aggressive
optimizations are possible)
〉 Additional protection countermeasures against side-channel attacks
〉 Provide support for other platforms (ongoing work for ARM
architecture)
〉 Provide additional guarantees when compiling C using CompCert
(ongoing work)
Manuel Barbosa, David Castro, Paulo Silva 14/14