# Tag Info

15

This question has many problems in the way it was asked, and clearly did not come after doing some investigation. However, since this seems to be a misconception that is spreading widely, I will relate to it. It is not true that the "crypto community" (whoever that is) believes that the NSA can break RSA. In fact, if Snowden taught us anything, it is that ...

9

Let me first answer your actual question (and then I'll proceed to answer something slightly different that I think will be informative and helpful). Your question asks whether it's possible to use only a DSA key. Technically speaking, this is of course possible. The reason is that a DSA key has exactly the same format as an ElGamal key. No one forces you to ...

8

The distinction is that ECDSA solves a problem that HMAC does not. If you need that problem solved, then you need to do ECDSA rather than HMAC; if you do not, then HMAC works just as well (and is a lot cheaper). With HMAC, here is what we have: we have an authenticator that has a secret key. It takes a message, and gives that (and the secret key) to the ...

7

The correct term for bytes to be signed is “message”. Generally, it does not really matter if a message to be signed is human readable or not. Sometimes, you may also find it mentioned as “digital message”… which practically is the same and merely extends the term to explicitly hint at the fact the message is digitally stored and/or processed. References ...

5

I'd say that most of the time the signature is accompanied by the certificate of the signer. This certificate contains the public key. Most container formats such as CMS (used in S/MIME, also known as PKCS#7) or XML digsig contain specific fields that may contain certificates - and usually do. When the certificate is received the Public Key Infrastructure ...

5

Deterministic signatures are safe in the random oracle model. Using HMAC_DRBG allowed me to rely on existing research on the safety of that construction and how close it comes to a "true" random oracle. If I had used any other "handmade" construction, then I would have had to provide extensive analysis on why it is secure. Being naturally lazy, I chose ...

5

Yes. Discrete-log based cryptosystems (e.g., El Gamal) have a similar property. It's "multiply", not "add", but that's just because that's the group operation for discrete-log based cryptosystems. More generally, I suggest you look at threshold cryptography. Threshold public-key cryptosystems are a class of systems that can achieve the sort of thing you ...

4

The ECDSA algorithm can't be used for encryption. It's not that there's no accepted way to do it, it's that it's simply not possible to do so. Likewise, RSA signing can't be used to encrypt (there's a mandatory hash in signing that you don't want in encryption). However, RSA signatures work similarly to RSA encryption; they aren't interchangeable, but ...

4

During verification you perform $u_1 = H ( m ) w \bmod{q}$ would return the same $u_1$ for multiple hash values. In general you only want one hash to succeed, even if it is unlikely that an adversary can generate a hash that equals $n q + H(m) w$. Of course this means that any hash with the leftmost bits identical to the org. hash is also acceptable, but ...

4

Assuming you manage to safely generate RSA keys which are sufficiently large, i.e. >= 2048 bits, no TLS configuration flaw on your side, and the lack of security bugs in the TLS library used by your server or the one used by the client user agent, I do not believe TLS_ECDH_RSA_WITH_AES_128_GCM can, at this point, be decrypted by surveillance agencies. ...

3

First to explain you, why you get 512-bit outputs from a 256-bit curve: The output is basically a point (x-coordinate is enough) and a message-dependant value, with the x-coordinate being expressed as integer. You can verify the signature by checking for a specific relationship between the point and the message-dependant value and the public key point. In ...

3

Sure you can do. There are many lattice attacks, using your second assumption, to ECDSA (which also applied to DSA). For instance see Smart and Howgrave-Graham and Shparlinski and Nguyen. All the lattice attacks base on finding small solutions (for the ephemeral key $k$ and the private key $a$) to the signing equation $sk-ra\equiv H(m)\pmod q.$ If you have ...

3

Generically speaking you can do this but you shouldn't. It may well be possible to perform specific calculations when a random number is used for both (I'll leave it to the more theoretically inclined to create a demo if this is possible for ElGamal / DSA). Another reason is that the single secret gets known then both keys/algorithms will be compromised. ...

3

An attack is described in Section 4.1.6 of the SEC1 document. Regarding xagawa's answer: The attack you describe is different from that described in Section 4.5 of the Blake-Wilson--Menezes paper. Specifically, their attack: (a) does not require knowledge of the secret ephemeral key $k$, and (b) changes the reference point $P$, which is not allowed in ...

3

In your particular case the order of the point divides $p-1$, this means that the embedding degree of your curve is 1. You should be able to apply the MOV attack to transfer your instance of ECDLP into an instance of DLP over $\mathbb{F}_{p}^*$. This would allow you to use the Index Calculus to solve your problem. As the Index Calculus is subexponential, ...

3

There is no standard way, and I think it's impossible to find universal agreement. However, it is pretty common to use lowercase letters for integers and uppercase letters for elliptic curve points. And it is very common to indicate as p the prime defining the field $\mathbb F_p$ over which the curve is defined. For the order of the curve you can use ...

3

To add to poncho's answer (since they beat me to it!), there are several advantages to choosing HMAC over ECDSA (or RSA) if you can get away with it: Insanely better performance: signing and verifying is much faster Much simpler implementation: this is important for security (even if you're not the one doing the implementation), since more complexity leads ...

3

The problem is that, imagine you sign a message $m$ using ECDSA and SHA-1 as hash algorithm. If an attacker manages to find a message $m'$ such as SHA-1$(m)$ = SHA-1$(m')$ then the computed signature for $m$ will be valid for $m'$. So the attacker can substitute $m$ for $m'$ while keeping the same signature value. The receiver who will try to validate the ...

2

ECC public keys are not random numbers. They are generated through a scalar multiplication of a random scalar with a known/public point on the curve, called Generator. The entropy lies entirely in the random scalar. Two public keys will collide if the random scalars collide. When you say "256bit ECDSA public key" you probably mean that your elliptic ...

2

There is a way to generate forgeries for (EC)DSA when the hash function is not one-way: Let $n$ be the order of the group, $P$ a generator, and $Q = aP$ for some secret $a$; Pick arbitrary $\alpha$ and $\beta$ $\in \{0, \dotsc, n\}$; $r = x \bmod n$, where $(x, y) = \alpha P + \beta Q$; $s = r \beta^{-1} \bmod n$; $h = s \alpha \bmod n$; Invert $H(h)$ to ...

2

In ECDSA, the message is never encoded as a point in the elliptic curve. Signing in ECDSA loosely works like this: \begin{align*} k &= \text{random}(0, n) \\ (x, \_) &= k \cdot G \\ r &= x \bmod n \\ s &= k^{-1}(H(m) + r \alpha) \bmod n \end{align*} $r$ and $s$ are the signature, and as you can see $H(m)$ is only ever used as an ...

2

ECDH is not for signing. Your sign method using ecdh does not look like any valid signature scheme I have ever seen, and is therefore likely wildly insecure. Note that the Q&A you link to is asking a very different question.

2

Typically, a message will contain some sort of identifying information of the sender, such as the From header of an e-mail. In any case, if the sender of the message is unknown, what's the point of using signatures at all? The purpose of a signature is to ascertain that the message was written and sent by its purported sender. The only way to be 100% ...

2

What is its signature length ? Depends on what algorithms you use, but with ECDSA the signature length is twice the length of the order of the base point. For P-521 that's 1042 bits, or 132 bytes when using whole bytes for each part. For E-521 it's 1038 bits or 130 bytes. How is it better ? The design criteria for E-521 are stated in A note on ...

2

You are probably aware of the existence of public key certificates. A certificate proves the authenticity of a public key, basically by signing the value of that public key (plus some data on the owner of that key) with a private key of some third party. This third party often is a central Certificate Authority (CA) that is trusted by both the sender and the ...

2

ECKEY object may contain: Group Private key Public key Both Group and Private key are needed to be able to calculate signature. It is most convenient to use generic ECKEY object (from API perspective), as it easy to e.g. convert between commonly used PKCS#8 PEM encoded EC private keys and ECKEY objects, and because just a BIGNUM would not be sufficient. ...

2

Trivial solution: generate a random $k$ as part of the private key and include $r$ as part of the public key. The verifier uses $r$ from public key, so the signer must use the same $k$ for every valid signature. The signer could create multiple related public keys and reuse $D_A$, but then, they might as well just create multiple key-pairs in the first ...

2

Well, it makes the discrete log problem easier; hence it makes recovering the private key easier The DSA private key is a value $k$, and his public key is the value $g^k \bmod p$. $g$ has order $q$; if $q = q_1 \cdot q_2$, then the attacker can compute $(g^k)^{q_2} = (g^{q_2})^k$. The value $g^{q_2}$ has order $q_1$, and so he can solve that problem in ...

2

Asn1parse to the rescue. Most of the overhead is from the base64 encoding and the PEM header and footer. The raw size of the compressed form ASN1 encoding is just 44 bytes for my dummy key. And 22 of those bytes are for the 161 bits of the actual public key. \$ openssl asn1parse -in compressed_public.pem -i -dump 0:d=0 hl=2 l= 42 cons: SEQUENCE ...

2

I'm not sure, but I guess that two different openssl version or just builds could be done with or without support of Elliptic Curves with unsecure security levels. Furthermore your curves have very low security level, and you shouldn't use them if security is a concern (and if it's not, you probably don't need ECDSA at all). However, your assumption that ...

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