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Charm: a framework for rapidly prototyping cryptosystems

Abstract

We describe Charm, an extensible framework for rapidly prototyping cryptographic systems. Charm provides a number of features that explicitly support the development of new protocols, including support for modular composition of cryptographic building blocks, infrastructure for developing interactive protocols, and an extensive library of re-usable code. Our framework also provides a series of specialized tools that enable different cryptosystems to interoperate. We implemented over 40 cryptographic schemes using Charm, including some new ones that, to our knowledge, have never been built in practice. This paper describes our modular architecture, which includes a built-in benchmarking module to compare the performance of Charm primitives to existing C implementations. We show that in many cases our techniques result in an order of magnitude decrease in code size, while inducing an acceptable performance impact. Lastly, the Charm framework is freely available to the research community and to date, we have developed a large, active user base.

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Notes

  1. Project webpage: http://charm-crypto.com.

  2. A dedicated module to support lattice-based cryptography is in preparation for a future release.

  3. Nor are we the first to import cryptographic operations into Python. See, for example, [37, 71].

  4. It is also well supported. Our experiments show that there have been significant performance improvements between Python 2.x and 3.x. Charm supports both versions for backwards compatibility with legacy applications.

  5. For consistency, group operations are always specified in multiplicative notation, thus \(*\) is used for EC point addition and \(**\) for point multiplication. This makes it easy to switch between group settings.

  6. For more scheme implementations, see http://jhuisi.github.com/charm/schemes.html.

  7. In practice, we first compile to bytecode, then execute. This reduces overhead for proofs that will be conducted multiple times.

  8. Clearly the verifier does not have access to the secret variables. We address this later in this section.

  9. In some cases, evaluation of a scheme depends on the scheme’s public key.

  10. The value \(r\) is typically a large prime.

  11. On a call to encrypt or keygen the adapter simply hashes an arbitrary string into an element of \(\mathbb{Z }_r\), then passes the result to the underlying IBE scheme. This technique and its security implications are described in [17].

  12. Naor [40] observed that adaptively-secure IBE can be converted into a signature scheme by using the IBE key extraction algorithm for signing.

  13. http://www.cs.auckland.ac.nz/~pgut001/cryptlib/.

  14. http://www.rsa.com/rsalabs/node.asp?id=2301.

  15. http://www.cryptix.org/.

  16. http://www.bouncycastle.org/.

  17. http://charm-crypto.com/Documentation.html.

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Correspondence to Joseph A. Akinyele.

Appendix

Appendix

See Figs. 8, 9, 10, 11, 12, 13.

Fig. 8
figure 8

A working example of how the API is utilized in a C application to embed a hybrid encryption adapter (see Fig. 9b) for any CP-ABE scheme such as the BSW07 [14] scheme shown in Figs. 11 and 12. We provide several high-level functions that simplify using Charm schemes. In particular, the CallMethod() encapsulates several types of arguments to Python such as: %O for Charm objects, %s for ASCII strings, %A to convert into a Python list, and %b to a binary object

Fig. 9
figure 9

a The entire IBE to signature adapter scheme [19]. b A hybrid encryptor for ABE schemes in Charm

Fig. 10
figure 10

Keygen in the Cramer–Shoup scheme [38]. We exclude group parameter generation

Fig. 11
figure 11

Setup and Keygen in the Bethencourt, Sahai, and Waters scheme [14]. We exclude group parameter generation

Fig. 12
figure 12

Encryption and decryption in the Bethencourt, Sahai, and Waters ABE scheme [14]. The Charm toolbox provides several utility routines that are shared by different ABE schemes

Fig. 13
figure 13

CL signatures [30] are a useful building block for anonymous credential systems. We provide a full scheme description and Charm code, but exclude group parameter generation

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Akinyele, J.A., Garman, C., Miers, I. et al. Charm: a framework for rapidly prototyping cryptosystems. J Cryptogr Eng 3, 111–128 (2013). https://doi.org/10.1007/s13389-013-0057-3

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Keywords

  • Applied cryptography
  • Protocols
  • Software
  • Privacy