There are different objectives when it comes to security. One objective is to make messages confidential. An OTP provides this in an information-theoretical secure way. Of course, the premise of OTP is that the key stream is fully random, which is hard to prove conclusively.
Other security goals could be maintaining message integrity: making sure that the message isn't tampered with or otherwise altered and message authenticity: making sure that the message that is received is the message that the sender wanted to send to the receiver. These goals are not covered by an OTP, as a message can be tampered with during transit or when in storage.
If an adversary can only listen and can not interfere with the message then we talk about an eavesdropper. As no interference is possible, it is generally not needed to secure the message to achieve integrity / authenticity. In that case an OTP is sufficient to protect the message, as it provides the confidentiality required. Trying to decipher a confidential message without changing the transport of the message itself or performing any actions involving the participants is called a passive attack.
However, for most systems, we have to assume that active attacks - such as man-in-the-middle attacks - are possible. In that case the attacker may try to tamper the message in transit, or send messages of his own to the receiver. If any invalid message is accepted then integrity and authenticity of the messages is obviously not achieved. One method of changing the messages is a bit flip attack, but it would also be possible to change the size of the message or tamper it in any other way. The bitflip attack is however powerful because it allows an attacker to flip any specific bit of the plaintext.
I've seen it in this group, and apparently there are a (small) number of voices on the 'net opining that those relying on OTP are essentially "amateurs" (or otherwise oblivious) to the possibility of "bit-flip" being a fatal flaw in OTP and they should NOT be used. I found these claims astonishing and was interested in learning more.
I don't see that in the articles you posted to. Yes, the articles pointed out the flaws of schemes that uses OTP's and the drawbacks of OTP's, but I don't see any name calling (calling somebody an "amateur" would be a strange way of name calling, a good amateur can be better than many professionals out there, if we take the terms by their literal meaning).
"Bit Flip" in the context of these claims being that the OTP message can be tampered with in transit- that there's no validation the recipient has received exactly the message that was originally sent. But even tampering doesn't imply that the attacker can read the text they tampered with sans a key. So as far as I can see at this point, the secrets still remain protected.
Correct. OTP's provide perfect confidentiality (well, except for the message length or timing, never forget that), so no attack can change that. Note that the system may still leak information about a tampered message though. For instance, if computation based on the received message may trigger specific errors, and those errors can be detected by an adversary, then plaintext oracle attacks may leak information about the contents of the message (any action that depends on the plaintext message may do that, of course).
Further, in order to tamper with the contents of the OTP encrypted message would imply that the attacker knew exactly "when", "where", "how" and most importantly "who" the message was being transmitted to. Those decrying One Time Pads as an insecure, flawed system are thin on details how an attacker would obtain this key these key pieces of data.
Of course they are; they must be thin on details as the methods of altering a ciphertext message in transit depend fully on the system. Unfortunately, there are many ways that active attacks are possible, and you ignore them at your peril.
Such a claim is bold and it contravenes accepted wisdom that OTP encrypted messages are secure & unbreakable- if the system is operated correctly of course. Am I missing something here?
I think you are naive if you think that it is easy to detect tampered messages without leaking data. Similarly, I think you incorrectly assume that active attacks are commonly not possible. These are serious issues and it is unwise to ignore them in the best of cases.
In the end, key management with one-time-pads is awkward at best and outright impractical and dangerous at worst. We've got secure schemes such as authenticated cipher modes (ChaCha20/Poly1305 and AES/GCM, for instance) that provide symmetric encryption that include small key sizes as well as integrity / authenticity of the message. These are more secure to use for most systems, certainly compared to schemes that just use an OTP (rather than an OTP combined with a MAC authentication tag, for instance).
Sure, it may be possible that some schemes are better off using an OTP. Those are however so uncommon that OTP should be considered a red flag when it comes to cryptography. It commonly means that the designer of the scheme doesn't know about or severely underestimates the limitations and drawbacks of OTP's.
The fact is that an OTP protects you against breaking of other symmetric cipher algorithms / implementations. That's currently not much of an issue for most ciphers. If you can make a conscious, well founded decision that an OTP is required to protect you against such attacks then go for it. But please be aware of the listed limitations and drawbacks of OTPs.