Signatures that outlive their algorithms.
How proof stays alive when the cryptography that created it ages.
A digital signature is only as strong as the algorithms that support it, and algorithms age. What is impossible to forge today may become feasible in ten or twenty years, through advances in computing power, mathematical discoveries or new technologies. Many documents, on the other hand, must remain provable for decades: a deed, a long term contract, a medical record. The question is how to make the proof outlast the cryptography that created it.
Algorithms have an expiration date
Recent history shows this clearly. The MD5 hash function, widely used in the 1990s, stopped being considered secure in the mid 2000s, when it was shown that collisions could be manufactured. Its successor, SHA 1, followed the same path: in 2017, researchers from Google and the CWI in Amsterdam published two different PDF documents with the same hash. Key sizes also fell short: 1024 bit RSA keys, common twenty years ago, are no longer accepted for new signatures.
The most talked about horizon is quantum computing, which in theory could break the most widely used public key algorithms. In 2024 NIST published its first post quantum signature standards, and the transition will take years. But the underlying problem is neither new nor the last of its kind: it is the normal condition of cryptography.
The central idea: renew before it breaks
The solution adopted by the standards is simple in principle. While an algorithm is still secure, a new timestamp, made with stronger algorithms, is added over the signature and its validation data. That timestamp proves that the original signature existed, and was verifiable, at a time when it was still trustworthy. Before the new algorithm ages in turn, another timestamp is added, and so on. The proof is kept alive in layers.
- At signing. The signature is produced with current algorithms and receives a timestamp.
- Shortly afterward. The certificates and revocation information are embedded in the document, so that validation does not depend on services that may disappear.
- Periodically. An archive timestamp made with current algorithms is added, covering everything that came before.
- Always before expiration. Renewal takes place while the most recently used algorithm is still secure. If it comes too late, the chain is broken.
The standard tools
- Long term signature formats. The ETSI standards for advanced signatures in PDF, XML and other formats define levels that incorporate validation data and archive timestamps. The highest level of the PAdES profiles, called B LTA, is designed precisely for this.
- Evidence records. RFC 4998, from 2007, describes how to protect large sets of documents with hash trees and renew them all at once, without touching each file.
- Algorithm catalogs. Bodies such as ETSI, in its specification 119 312, and national cybersecurity agencies publish which algorithms and key sizes are considered adequate, and until when.
Crypto agility
For all of this to work, systems must be able to switch algorithms without being rebuilt from scratch. This is called crypto agility: knowing where each algorithm is used, being able to replace it through configuration, and keeping an inventory of what needs to be renewed. Organizations that build it before they need it get through each transition without disruption.
- Which documents must remain provable for more than ten years?
- Are they signed in a format that supports archive timestamps?
- Who monitors the algorithm catalogs and decides when to renew?
- Can the system change algorithms without a full migration?
Every cryptographic algorithm ages. Proof can outlast cryptography if validation data is embedded and archive timestamps with current algorithms are added before the earlier ones stop being secure. The standards already solve the how; what is needed is the discipline to renew on time.